Well, if the tanks can't handle an HOB
due to the size of their lip, and canister filters are out because of the cost, you really don't have very many options.
If I were you, I might try to go with a combination of 1 corner ("box" or "internal") filter and one good-sized sponge filter in each tank. The Hydro-Sponge filters have a good reputation; I currently use those in my freshwater shrimp tanks and they do a fine job. This one
would probably be the size for tanks you are describing. I'd personally stay away from under-gravel filters, they can be a pain when it comes to maintenance and sometimes can make your life much more difficult when it comes to keeping the substrate clean.
I don't know the first thing about the particular species of fish you are dealing with, but when it comes to breeding fish in general, the things that are going to help with having success would be:
- keeping your water clean--ammonia & nitrites zero, and nitrates at a minimum
- water parameters as close as possible to what the fish have in their native habitat (pH, temperature, GH/KH hardness, TDS)
- tank setup as close to their natural biotope as possible (do they come from waterways with lots of plants or very few? Can you grow in your tank native species of plants, or others that are very similar in appearance? Are they used to rocky bottoms or sandy? etc.)
- An understanding of whether or not adults will cannibalize their own fry; if they will, then you need to have plans to remove the adults to different tanks after the fry are born/hatched
- live food, and lots of it. While some species of fish can and will breed on a diet of commercial fish flakes, many fish are less likely to breed (or will not breed at all) without a diet of live foods
If the fish you are getting are actually tank-raised (and have never been in the wild), then when it comes to water parameters you would want to make sure yours were reasonably close to the tanks from which they came.
Good luck with your project. Please keep us informed.